Semiconductor mounting techniques have accomplished remarkable progress in recent years. TAB, in particular, is under active development in order to respond to the current requirement for high-density mounting technique, because TAB enables formation of conductor pattern at a very high density, making it easy to connect multiple pins, and moreover TAB enables simultaneous bonding of all leads with semiconductor chips (gang bonding) without using any wire.
There are two kinds of tapes for TAB, i.e. a tape for TAB having a two-layered structure and a tape for TAB having a three-layered structure. The tape for TAB having a three-layered structure (hereinafter abbreviated as three-layered tape for TAB) is generally a laminate obtained by laminating a conductor foil (e.g., copper foil) with a heat-resistant resin film using an adhesive and has an adhesive layer of poor heat resistance; hence, the three-layered tape for TAB has been unable to fully exhibit the superior properties even when there is used, as the resin film, a polyimide film superior in properties such as heat resistance, chemical resistance and the like.
Meanwhile, the tape having a two-layered structure for TAB (hereinafter abbreviated as two-layered tape for TAB) generally has excellent heat resistance as a tape for TAB because the base film has no adhesive layer; however, the two-layered tape for TAB has found only very limited applications because of difficulty in production. That is, the two-layered tape for TAB is produced by two processes, i.e., the one process which comprises forming a conductor layer on a heat-resistant film made of a polyimide or the like by a thin-film-forming technique such as sputtering, vapor deposition, plating or the like, and then applying an etching treatment to the base film and the conductor layer to form desired holes (device holes, sprocket holes) and a desired conductor pattern, and the other process which comprises coating a varnish of a heat-resistant resin (e.g., polyimide) directly on a conductor foil (e.g., copper foil), drying the coated varnish to form a two-layered base board, and then applying an etching treatment to the resin layer and the conductor foil to form desired holes and a desired conductor pattern. In the former process, however, there is a problem that a rolled foil of excellent flexibility, a Fe--Ni alloy foil of excellent modulus, etc. cannot be used; moreover, the base film holes must be formed by an etching method, resulting in significantly reduced productivity as compared with the case employing the punching method applied to three-layered tapes for TAB. Meanwhile in the latter process, it is necessary that the linear thermal expansion of the resin used be nearly the same as that of the copper foil (conductor layer) in order to prevent formation of curls and wrinkles; however, such resins are generally rigid and have excellent solvent resistance, making it difficult to apply alkali etching in many cases and requiring use of dry etching (e.g. excimer laser) in order to form holes in the film layer, which results in disadvantageous productivity and cost.
Meanwhile, two-layered flexible materials for printed circuit, having no adhesive layer are finding increased applications, as electronic appliances become smaller and lighter. Similarly, film cover layers having no adhesive layer are in active development. Further, not only conventional flexible materials for wiring but also materials having a supporting film with holes, such as the above-mentioned carrier tape for TAB have found increased applications recently.
In the two-layered flexible base board for printed circuit, obtained by a process which comprises coating a polyamic acid on a conductor foil, followed by curing, there is no problem caused by an adhesive layer; however, the film properties are still inferior to those of ordinary stretched polyimide films.
Also in production of an adhesive layer-free cover layer for a two-layered flexible material for printed circuit, it has been attempted to use, for example, a process which comprises coating a polyimide precursor ink directly on a two-layered flexible material for printed circuit by a screen printing method or the like, followed by drying. In this process, however, use of a polyimide precursor varnish as an ink requires addition of a filler or the like for increased thixotropy, which makes it difficult to obtain balance between the filler-free polyimide film layer (base material) and the filler-containing polyimide cover layer. As a result, it has been impossible to obtain a flexible printed circuit board with a cover layer, having sufficient folding resistance.
Each of these conventional processes has drawbacks in heat resistance, adhesion strength, processability and chemical resistance. First, the adhesive layer-containing three-layered flexible material for printed circuit has a drawback that since the adhesive layer has low heat resistance, the heat resistance of the flexible material for printed circuit is determined by the heat resistance of the adhesive layer even when a polyimide is used as the supporting film. When a conductor layer is formed by vapor deposition or sputtering, there is a drawback in that the adhesion strength between the supporting film layer and the conductor layer is insufficient. In the case of casting method a polyamic acid is coated directly and then dried to form a thick film, and the stress due to the contraction caused by imidization is larger than the supporting strength of the conductor layer, producing large curls during drying. When a polyamic acid solution is directly coated on a conductor layer and dried a plurality of times, followed by imidization, the curls produced during these steps are reduced; however, since the film portion close to the conductor layer and the film portion distant from the conductor layer have different thermal histories, the imidization degree and residual solvent amount in film each differ depending upon the film portion, making large the curls, dimensional change, etc. of the film after etching of the copper foil. When there is used a laser, in particular, an excimer laser, excellent fine processing is obtained and damage to copper foil is small; however, there are problems that the strong imide bond must be cleaved, resulting in long processing time, low productivity and high running cost. Also, a polyimide having a molecular structure capable of being subjected to alkali etching after imidization has problems that it has somewhat low solvent resistance, that use of a strongly alkaline solution as an etching solution involves serious danger and that disposal of the waste solution is not easy.
An object of the present invention is to provide a process for producing a two-layered tape for TAB at high productivity without reducing heat resistance, alkali resistance, solvent resistance and electrical properties inherently possessed by two-layered tapes for TAB having no adhesive layer.
A further object of the present invention is to provide a process for producing a two-layered tape for TAB which is free from curls and which has good adhesion between the copper foil and the film.
Another object of the present invention is to provide a process for producing a two-layered tape for TAB which enables excellent registration and formation of fine lines and which enables high-density wiring.
A still further object of the present invention is to provide a process for producing a stretched polyamic acid film having a desired thickness and, optionally, holes, which film enables production of a two-layered flexible printed circuit board having no adhesive layer, which board has a polyimide layer having the same folding resistance, alkali resistance, solvent resistance, heat resistance, electrical properties and mechanical properties as possessed by ordinary stretched polyimide films (such a polyimide layer has been unobtainable by the conventional varnish casting process) and which board enables use of any conductor foil (e.g. rolled foil which cannot be used in the vapor deposition process).
A still further object of the present invention is to provide a flexible base board for printed circuit or a flexible printed circuit board with a cover layer film, both enabling high-density wiring and having very high heat resistance, by laminating a stretched polyimide film with desired holes to a conductor foil or a two-layered flexible base board with no adhesive layer and then conducting imidization to form, directly and using no adhesive, a polyimide insulation cover with holes without reducing the folding resistance, alkali resistance, solvent resistance, heat resistance, electrical properties and mechanical properties inherently possessed by polyimide films.